Check the Windows power management options, there may be an energy saver mode that is restricting your clocks. If that fails to work, you can download an overclocking application such as MSI Afterburner and bring the clocks back up to the manufacturer spec.

it depends on the game. some 3d games does not need all the raw power available from the gpu to run optimally. in such a case the gpu will clock down itself properly to save power and reduce the core temperature. it's parts of the power management of the card but you can force the card to run at it's full clock at all time if you want to. the changes can be made via nvidia control panel.

anyway before playing around with control panel setting you should check if this behavior also happen in other games you play or you can simply run graphical test benchmark such as heaven unigine and see if the clocks stays that way

from the control panel go to manage 3D setting. there are to tabs which is global setting and program setting. the former will affect all program that use the gpu and the later will only affect the program you specifically choose. look for power management mode and change the setting from adaptive (default) to prefer maximum performance. btw i suggest you to run something like furmark. in normal circumstance the clock should rise as it should be under furmark heavy load

Well if you tried eveything then I would RMA it ASAP before the window closes. You only have the one 550 right?I have two and in SLI my second one uses about 53% while playing.Is the slot on your MOBO a PCIe x16 or x8 or x4?If it is a times 4 then it would run very slow.